The present invention generally relates to communication systems, and more particularly to a duplexed communication system having a switching system which switches from a working system to a protection (or standby) system by detecting an abnormality in the systems.
In a communication system, a duplexed system configuration is employed at various parts of high-speed and low-speed parts which multiplex and demultiplex signals. If an abnormality is detected in a working system, the duplexed system is quickly switched to a protection system so as to improve the availability of the communication system. Accordingly, it is desirable in such a communication system to precisely separate the part of the working system where the abnormality was generated and to switch only the part where the abnormality occurred.
FIG. 1 shows an example of a conventional duplexed communication system. This communication system is the so-called synchronous optical network (SONET) system. In FIG. 1, transmitters are labelled "TX" receivers are labelled "RX" alarm detectors are labelled "M" switches for the lines are labelled "SW", switch controllers for the switches SW are labelled "SC" multiplexers are labelled "MUX" and demultiplexers are labelled "DMUX".
The SONET system is divided into a high-speed common part and a low-speed channel part CH, and the parts are duplexed, that is, have the redundant configuration, so as to cope with failures. The section between the transmitter TX and the receiver RX of the high-speed part are respectively referred to as a "section". The section between the multiplexer MUX and the demultiplexer DMUX is referred to as a "line". In addition, the section between the channels CH and CH of the low-speed part is referred to as a "path". The failure detection in each of these sections can be made by checking specific parity check information.
FIG.2 shows the STS-1 frame structure employed in the SONET system. One frame 100 from the STS-1 includes an overhead part 101 amounting to 9 lines.times.3 bytes, and a payload (information transmitting) part 102 amounting to 9 lines.times.87 bytes. The overhead part 101 includes a section overhead 101a and a line overhead 101b. The payload part 102 includes a path overhead 102a.
The section overhead 101a includes frame synchronizing byte information A1 and A2, and parity check byte information B1. At the part related to the section S in FIG. 1, the frame synchronization error is detected from the abnormality of the frame synchronizing byte information A1 and A2, and the bit error rate between the transmitter TX and the receiver RX is detected from the abnormality of the parity check byte information B1. The line overhead 101b includes parity check byte information B2. In the part related to the line L in FIG. 1, the bit error rate between the multiplexer MUX and the demultiplexer DMUX (excluding the section S) is detected from the abnormality of the parity check byte information B2. On the other hand, the path overhead 102a includes parity check byte information B3. In the part related to the path P in FIG. 1, the bit error rate between the channels CH and CH (including the errors caused within the line L) is detected from the abnormality of the parity check byte information B3. Hence, the parity check of the SONET system has a hierarchical structure, and the part related to the section S, for example, is not aware of the failure generated in the line L or the path P.
Returning now to the description of FIG. 1, attention is drawn to a channel CH1 on the output side, for example. Transmitters 21a and 21b respectively transmit the same transmission signal to receivers 23a and 23b. In this state, an alarm detector 24a outputs an alarm signal AL1 when the alarm detector 24a detects an alarm state of a received signal at the receiver 23a such as the bit error rate exceeding a predetermined value. Similarly, an alarm detector 24b outputs an alarm signal AL2 when the alarm detector 24b detects an alarm state of a received signal at the receiver 23b. If it is assumed for the sake of convenience that the receiver 23a forms the working system, a switch controller 25 receives the alarm signal AL1 from the alarm detector 24a but receives no alarm signal AL2 from the alarm detector 24b, for example. In this case, it may be judged that the failure simply exists only between the transmitter 21a and the receiver 23a, and it is possible to appropriately switch a switch 26 from a contact a to a contact b, that is, from the working system to the protection system. The switching can be made similarly if the receiver 23b forms the working system.
On the other hand, if a failure is generated at a demultiplexer 41a, for example, a switch controller 45 receives an alarm signal AL1 from an alarm detector 42a and receives no alarm signal AL2 from an alarm detector 42b. Hence, it is possible to appropriately switch a switch 46 from a contact a to a contact b, that is, from the working system to the protection system. However, it inevitably takes time for the switching operation to be completed from the time when the failure is generated, and a considerably amount of deteriorated transmission signal is transmitted as it is to the channel part during this time. As a result, the alarm signals AL1 and AL2 are generated approximately at the same time and at predetermined intervals for a plurality of times at the alarm detectors 24a and 24b of the channel part.
In such a case where the alarm signals AL1 and AL2 are generated approximately at the same time and at predetermined intervals at the alarm detectors 24a and 24b, the switch controller 25 first accepts the alarm signal AL1 from the alarm detector 24a of the working system and switches the switch 26 from the contact a to the contact b, and thereafter accepts the alarm signal AL2 from the alarm detector 24b which now forms the working system and switches the switch 26 from the contact b to the contact a.
Furthermore, if a failure is generated between a transmitter 11a and a receiver 13a of the high-speed part, for example, it is possible to appropriately switch from the working system to the protection system at a switch controller 15. But in this case, the transmission signal which is deteriorated between the transmitter 11a and the receiver 13a is distributed as it is to each of the output ports 1 through 4 of the demultiplexers 41a and 41b. For this reason, the complex switching control similar to that described above was frequently carried out in the switch controller 45 on the downstream side and in the switch controllers 25 and 35 of the low-speed channel part.
Therefore, in the conventional SONET system, there was a possible problem in that the switching of the duplexed system is frequently carried out not only in the part of the system where the abnormality actually occurred but also in other parts of the system. This is a problem common to the general communication systems which carry out multiplexing and demultiplexing between high-speed and low-speed lines and switch one of N working systems to a protection system by detecting the alarm of the transmission signal at each part of the high-speed and low-speed lines, where N is an arbitrary integer. Of course, the communication system may have one protection system with respect to each working system.
In order to prevent an erroneous operation of the switch which is originally unrelated to a failure which is generated on the upstream side of the switch, various methods have been proposed.
According to a first method, a guard time is set in a switch controller. This guard time is longer than a time it takes for the part in the upstream side to start a normal operation from a time when a failure actually occurs by detecting this failure and switching the working system to the protection system. Hence, the part on the downstream side will not accept an alarm signal which is received within the set guard time, so as to prevent an erroneous switching in the downstream side part. But this first method, there was a problem in that the switching time of the downstream side part becomes considerably long.
On the other hand, a second method prevents the erroneous switching by prohibiting the switching in the downstream side part while an alarm signal is generated in the upstream side part. FIG. 3 shows an essential part of a conventional communication system which employs this third method.
In FIG. 3, a receiver 101a receives a signal from a working system while a receiver 101b receives a signal from a protection system. A switch 102 is switched in response to a control of a switch controller 103, and the signal output via the switch 102 is received by demultiplexers 105a and 105b. The switch controller 103 carries out the control based on alarm signals from the receivers 101a and 101b. A switch 106 is switched in response to a control of a switch controller 107, and the signal output via the switch 106 is received by channel parts 109a and 109b. The switch controller 107 carries out the control based on alarm signals from the demultiplexers 105a and 105b. A switch 110 is switched in response to a control of a switch controller 111. The switch controller 111 carries out the control based on alarm signals from the channel parts 109a and 109b.
In addition, the switch controller 107 prohibits the switching of the switch 106 based on the alarm signal corresponding to one of the receivers 101a and 101b selected by the switch 102. Similarly, the switch controller 111 prohibits the switching of the switch 110 based on the alarm signal corresponding to one of the demultiplexers 105a and 105b selected by the switch 106. Hence, the erroneous switching is prevented by prohibiting the switching in the downstream side part while the alarm signal is generated in the upstream side part.
However, this second method has no effect if the alarm detection time of the downstream side part is shorter than the alarm detection time of the upstream side part or, the alarm recovery time of the downstream side part is longer than the alarm recovery time of the upstream side part. Hence, it is necessary to employ the first method together with the second method. Furthermore, there is a problem in that it is essential to transmit the switching prohibiting information from one stage to another, and this second method is not suited for practical use if the plurality of stages are relatively distant from one another.
According to a third method, the erroneous switching is prevented by limiting the alarms of the downstream side part to items unrelated to the failure generated in the upstream side part. But in this case, there is a problem in that it is impossible to obtain a sufficiently high failure detection capability.